Field of the Invention
The present invention relates to an electricity storage system and a method for controlling the electricity storage system.
Background Art
In recent years, an electricity storage system using a lithium-ion battery is proposed, in response to a growing environmental awareness. When the lithium-ion battery is continuously charged and discharged, ununiform dispersion of lithium ions progresses such that an internal resistance increases. Therefore, the lithium-ion battery has shortcomings in that a battery performance is degraded.
Ununiform dispersion of the lithium ions progresses due to variations in distribution of electrolytes in a positive electrode and a negative electrode, and generation of a region in which a sufficient battery reaction is produced and a region in which an insufficient battery reaction is produced.
JP-A-2009-44851 discloses technology that reduces degradation in a battery performance through continuous charging and discharging. JP-A-2009-44851 discloses technology in which, in a power system of an electric car including a plurality of secondary batteries which are electrically connected to a load in parallel, in a case where dischargeable electric power of each secondary battery is higher than a required output, a secondary battery having a predetermined discharge time longer than a predetermined time of the secondary batteries is deactivated not to perform discharge, and required power by the load is output from the rest of the secondary batteries. According to JP-A-2009-44851, the secondary battery is deactivated, and thereby unevenness in density of lithium ions is reduced. Thus, it is possible to have less unevenness in charges inside the battery such that it is possible to reduce degradation of the battery.
JP-A-2001-231178 discloses technology in which, in an assembled-battery control apparatus that controls an assembled battery including a plurality of secondary batteries which are connected in series, voltages of batteries are detected and capacitance is adjusted such that a difference in voltages of the batteries is small during deactivation of a battery. According to JP-A-2001-231178, capacitance adjusting means adjusts the capacitance such that the difference in voltages of the batteries is small during the deactivation when the difference is produced due to a difference between an internal resistance of the plurality of the secondary batteries and a charging current thereof during charging, and thus it is possible for the batteries to have uniform voltages, respectively.
JP-A-2013-27243 discloses technology in which battery packs connected in parallel are deactivated, and thereby degradation in cycle characteristics of batteries is reduced. JP-A-2013-27243 discloses that an electrolyte extruded from a negative electrode during charging returns to the negative electrode during deactivation, and thus variations in the electrolyte in the negative electrode, that is, variations in lithium ions, are eliminated. In JP-A-2013-27243, a deactivation time is set, and thereby uniform reactions of lithium ions in electrodes are performed such that it is possible to reduce battery degradation.
The technology disclosed in JP-A-2009-44851 is used in a power system for an electric car, and thus only discharge is controlled. However, in an industrial electricity storage system or an electricity storage system for power stabilization, not only the discharge control but also charge control needs to be performed. In addition, a state of charge (hereinafter, referred to as SOC) at the time of deactivation affects degradation of a battery; however, in JP-A-2009-44851, no attention is paid to the SOC during the deactivation and the SOC during the deactivation is not considered at all.
In JP-A-2001-231178, since an entire battery system is deactivated, a percentage (activation ratio) of batteries performing charge or discharge to the number of batteries is 0. However, in the industrial electricity storage system or the electricity storage system for power stabilization, the battery system needs to operate at a high activation rate over a long period of time such as 24 hours a day and 365 days a year. When the activation rate is lowered, operating costs of the electricity storage system are increased. Hence, it is not possible for the technology disclosed in JP-A-2001-231178 to be applied to a system in which continuous activation is required at a high activation ratio.
JP-A-2013-27243 discloses only a method for deactivating a battery pack, and a temperature of a battery during deactivation is not considered at all. In addition, in JP-A-2013-27243, the SOC during the deactivation is not completely considered.